During the process of coalification, a coalbed, under pressure and temperature, generates gases as well as a cleat (natural fracture) system. The cleat or fracture system is what allows gas and other fluids to flow from high flow potential to low flow potential areas in the coalbed. In the petroleum industry, the fluids of commercial interest are generally hydrocarbons, particularly methane. In areas where the coal is very well cleated and has good permeability, a vertical well can often provide for good recovery of the coalbed gases because of the high flow capacity of the reservoir(s). Cavitation completions can further enhance recovery in these wells. In lower permeability areas, vertical wells typically have to be fracture-stimulated for commercial production, and recovery efficiency is still commonly very poor because of low flow capacity.
This invention relates to a technique for drilling into coalbed methane formations and more particularly to drilling horizontal boreholes into coalbed methane yielding formations using a gas or mixture of gases as the drilling fluid.
Coalbed methane reserves of the Fruitland formation in the San Juan basin of northwest New Mexico and southeast Colorado were only recently tapped extensively as a commercial project. In the rush for companies to develop acreage and qualify wells for lucrative tax credits, many marginally economic wells were drilled and completed. Whereas some areas of the basin have coal seams with good permeability, providing for completions which yield high rates of return on investment, many areas have relatively low permeability and are not yielding good rates of return.
The original gas-in-place estimate for the Fruitland coalbeds is over 60 TCF, but only a small percentage of this reserve will be recovered from existing completions. In the areas of the basin which have low rate coalbed methane wells, significant upside potential exists if horizontal drilling in these formations can be effectively accomplished.
Fruitland formation coalbeds are generally high-volatile bituminous type A or B coals, with the majority of the lower rate coalbed methane wells completed in the less mature type B coals. These particular coalbeds exhibit a pattern of increasing maturity from the southern to the northern areas of the San Juan Basin as documented by published maps of vitrinite reflectance (R.sub.m or R.sub.o) data which range from less than 0.5 (sub-bituminous) to greater than 1.5 (low volatile bituminous). Vitrinite reflectance is a commonly used geological method for estimating the thermal maturity of organic material. The technique for determining this parameter involves measuring a reflectance characteristic of vitrinite material in the coal with R.sub.m being a mean reflectance value and R.sub.o being an interpretive number that is derived from a hystogram or plot of values wherein scattered data that is not representative of the overall character of the material is removed. Lab reports of these measurements are typically given as R.sub.o, which is more representative of the true character of the reservoir material. Vitrinite reflectance measurement is described in more detail by Ting F.T.C. (1991) "Review of Vitrinite Reflectance Techniques and Applications", Organic Geochemistry, Vol. 17, pp. 269-270 and by Kilby W. E. (1991) "Vitrinite Reflectance Measurement Same Technique Enhancements and Relationships", International Journal of Coal Geology, Vol. 19, pp. 201-218. A transition from high permeability to low permeability coal is coincident with a vitrinite reflectance of about 0.78 R. The majority of the low rate coalbed methane wells are located in areas where R.sub.o is less than 0.78 and the coals are ranked in the high volatile B bituminous or medium volatile bituminous grades. The Fruitland coalbed reservoirs are naturally fractured (cleated), containing both face and butt cleats as well as joints. In areas of higher permeability (i.e. generally coals with high volatile A bituminous rank or greater, (R.sub.o &gt;0.78)), properly completed vertical wells communicate effectively with the cleat system and are capable of efficiently draining the methane resources. In areas with lower permeability (i.e. coals with high volatile B bituminous rank or lower, (R.sub.o &gt;0.78)) not only is the overall effective permeability lower, but the anisotropy is greater, resulting in vertical well completions which are not efficiently producing the methane resources. Most of the wells in these low permeability areas have been fracture stimulated in an attempt to improve the production rate of the well but the results have been disappointing.
Basic rock mechanics concepts can be used to determine what orientation an induced fracture will assume. In the Fruitland coal seams, the orientation will be parallel to the face cleat system. Because of the anisotropy which exists, the propped fracture, by paralleling the higher permeability face cleats, does not maximize the production potential of the coal seams. Additionally, there is evidence that the induced fractures are inefficient because of apparent damage to the near-fracture area caused by compression of adjacent face cleats, swelling of in-situ clays, plugging by fluid additives, and/or swelling of the coal by water. Data and analyses in recently published literature indicates that the optimal completion of a vertical coal seam well is a cavitation completion or a completion which utilizes multiple fracture stimulations which may eventually orient perpendicular to the face cleats if the current stress orientations are favorable. In summary, it is generally believed that the current vertical well completions in the low permeability coal seams are not optimally drilled or stimulated.
Attempts to stimulate production from coalbed formations have included such techniques as (1) cavitation as shown in U.S. Pat. No. 4,305,464, (2) fracture-stimulation with various fluids and slurries, (3) cavitation of an open hole section by injection for example of air into coal followed by a rapid release (4) high pressure injection of a gas followed by rapid release of pressure to improve near-wellbore permeability as shown in U.S. Pat. No. 5,014,788, (5) horizontal drain holes, etc.
Induced hydraulic fractures in coal reservoirs are less effective than desired for the following reasons: (a) Hydraulic fractures do not cross-cut face cleats that are the most permeable pathways for fluid flow. Test data suggests that near wellbore permeability is less than that of pre-existing natural fractures located at greater distances from the well; (b) hydraulic fracture emplacement may cause increased horizontal stress and cleat aperture decrease with permeability decrease in the reservoir adjacent the induced fracture. To accommodate the volume of induced fractures, face cleats may be compressed distances on the order of 50 feet from the induced fracture with corresponding reduced permeability of one fourth to one tenth the original face cleat permeability; (c) the effective length and conductivity of the induced hydraulic fracture may be much less than designed due to complex induced fracture geometry and lithologic variation; (d) fracture fluids used to carry the proppant cause formation damage that reduces near permeability; and (e) hydraulic fracture gels may not break completely to leave residue that may plug cleats.
It is therefore an object of the present invention to overcome the problems associated with the development of low permeability, high anisotropy coalbed formations by using new and improved drilling techniques.
It is further the object of this invention to utilize gas or a mixture of gases as a drilling fluid medium for drilling and completing horizontal coalbed methane wells.
It is a still further object to optimize the natural permeability by drilling underbalanced and orienting the drilling direction to maximize intersection of the borehole and face cleats in the formation.